In a networked environment such as the Internet, it is often desirable to pair one or more supplemental documents, such as advertisements (or “ads”), with a requested document, such as a web page. A document viewer's experience may be enhanced by supplemental content that is of interest to the viewer. For example, a requested web page may be displayed along with one or more advertisements provided as supplemental content. These ads are selected based on a variety of criteria, often with the end goal of soliciting a reaction such as exploration of the document via a click-through and, in some cases, the conversion of the click-through into a transaction for value.